Prostate cancer (PC) is the second most common cause of cancer related deaths in men in the United States, and is one of the leading causes of sickness and death in men in the U.S. and Western Europe [8,9]. Despite this high prevalence, the molecular mechanisms of PC progression still remain largely unknown, due in part to heterogeneity during tumor development [10]. The five year survival rate of PC is almost 100% if diagnosed early. However the rate drops to ˜30% if diagnosed late or once it has metastasized to distant organs [69]. Localized primary PC is treated by prostatectomy or radiotherapy, but often patients develop metastatic disease. PC is initially androgen-dependent, making androgen deprivation therapy (ADT) the first line of defense in combating the disease [11]. Though this treatment initially reduces tumor size, almost all patients eventually develop androgen-independent prostate cancer (AIPC), which is resistant to this primary form of therapy and is ultimately lethal [11-13]. Thus, determining the mechanisms that contribute to AIPC is critical to develop novel therapies for this advanced form of PC.
Insertional mutagenesis screens using replicating retroviruses have identified many genes that contribute to cancer initiation and progression and have greatly improved our understanding of carcinogenesis (reviewed by Uren et al. [1]). These screens identify genomic loci which contain proviral integration sites that are identified from different tumors, called common insertion sites (CIS). These CISs occur because integrated retroviruses dysregulate nearby genes by a variety of mechanisms, and clones with provirus insertions near dysregulated genes that provide a selective advantage become enriched [2].
To date, the majority of insertional mutagenesis screens have utilized replicating gammaretroviruses (γRV) or transposons which have several limitations. Screens that use replicating retroviruses are limited to tissues and cell types that are permissive for replication of the virus. Because of this, the majority of screens have been performed in mouse hematopoietic cells or mouse mammary cells using replicating γRV vectors. Transposons allow mutagenesis of essentially any tissue and have expanded the use of mutagenesis screens. However, a major drawback of transposon approaches is the time it takes to generate the germline transgenic or knockout lines used, and to combine multiple alleles into the same background [3]. Another limitation of transposon mutagenesis is that multiple transposition events complicate the identification of causative mutagenic events [3].
By contrast replication-incompetent retroviruses do not replicate after integrating into the genome, and therefore do not introduce additional insertions. This reduces passenger insertions. Recent studies have used replication incompetent retroviral vectors as mutagens to identify driver genes involved in the initiation and progression of leukemia, liver, breast, pancreatic, and PC [4, 70].
High-throughput gene expression studies [40-42], comparative genome hybridization studies [43,44], and proteome studies [45-47] have previously been used to identify genes that are altered/mutated and differentially expressed between androgen sensitive and androgen insensitive PC. However, a major challenge in these high-throughput studies is differentiating driver mutations that cause cancer from passenger mutations that do not significantly contribute to the course of disease. Identifying the driver genetic mutations responsible for AIPC is critical to improve the prediction of recurrence and to contribute new therapeutics to increase the life expectancy of PC patients.